Precious Stones Cut
The cut, which is basically the composition of angles, the outline (shape) and facet arrangement of a polished gemstone is not evaluated according to the strict parameters typically applied to diamonds. Coloured gemstones first of all are always cut for optimization of colour. Nonetheless the accuracy of cutting to increase light return is becoming more important and a whole school of modern artistic gem cutters has since risen. Still modern taste will never override the laws of economics and allow for poor weight retention. The right balance between weight, beauty and profits always has to be struck. This is particularly true for very fine and valuable material, but does not mean unsigthly ‘potato shapes’ are acceptable.
Precious stones are broadly judged for harmonius proportions (principally width and depth), an elegant outline, good symmetry, and an adequate facet and polish quality.
A stone should not be too shallow as to avoid any windowing which will make the stone lifeless due to insufficent refraction and reflection. Coloured stones also need to be deeper due to their lower specific refractive indeces. However overly deep stones showing blackening and extinction as well as an overproportionate pavillion (‘belly’) are not acceptable. A good ‘cross-sectional’ profile also ensures that a stone does not face-up too small considering its actual weight. The ‘rule of thumb’ says that the crown should contain approximately 30% to 40% and the pavillion 60% to 70% of the mass.
The right combination of colour, ‘life’ and beauty in precious stones is further achieved by opting for the most appropriate facetting style according to the specific optical properties and transparency of a gem material. This will either be a brilliant (star) style, a mixed or step cut. Coupled with the right proportions the appropriate style promotes the greatest possible light return. As much light as possible must enter a stone, reflect internally and be refracted back to the observer. The final processing stage includes fine polishing to obtain a superior lustre. A well cut gemstone will usually have a brilliancy of at least 70 percent.
It is important to carefully check and orient gemstones for colour zoning. Furthermore many gems are dichroic (two-coloured) or trichroic (three-coloured), exhibiting different hues in different crystallographic directions. When cutting such stones a so called multicolour effect may result. Whilst rare and only seen in very few natural fancy coloured diamonds of mixed colours, it is common in gems. Depending on the actual colour ideal that has been set by cultural standards for a specific type of precious stones, this can either be advantageous or undesirable. In primary hued gems such as rubies, sapphires and emeralds this effect is generally less appreciated. This complex array of factors has definite implications towards the planning of viually pleasing stones.
The classic cutting styles for coloured gems are emerald cuts, cushions, ovals, rounds and pears.